What I'd call quite serious ones, ...
HCx thread on groundspeak
Colorado thread on groundspeak
have installed the available updates and I have never noticed such discrepancies ...
All these fora seem to breed
paranoia. 2 or 3 people post their bad experiences and the silent majority remain ... silent. Then someone subjects the unit in question to close scrutiny (as opposed to real-world usage) and lo - there is a measurable 'issue'.
(And the people on Groundspeak are, mainly, concerned with getting a static 'fix' - where we are more interested in following a route - as hazeii says, different GPS models are optimised in different ways.)
FWIW I now have a Legend C, Cx and, as of 3 weeks ago, a HCx (Sheila and I use one each and we carry one as a spare between us) and I don't see any significant difference between them in use, they all seem to me to work very well (though that doesn't stop me having a wish list as long as my arm).
The x models have a slightly brighter/contrastier screen and of course the memory card (which is a big plus) - the HCx model gets a fix faster than the others if you time them side by side, but its only a matter of 3 or 4 seconds, not significant in practice.
They all perform well on the bike under tree cover - again the H is measurably better but there is no practical difference at cycling speeds. They all have battery runtime which is 'good enough' ie 24h+ though I believe the older C model is the best here.
Both the older ones have been known to switch off due to road vibration - but not recently, with a bit more attention to the way they are mounted on the bars - I don't expect the new one to be any different.